The invention offers a solution to the phenomenon known as carburetor icing associated with this type of carburetor. Carburetor icing has been a problem for many decades but the mechanism for ice formation in a throttle plate carburetor does not seem to have been fully understood. The inventor believes the mechanism for ice formation to be as described below.
In this type of carburetor, a fuel mist issues from the jet. Normally the fuel is atomised by means of a venturi and the air flow through the carburetor, to set up the required fuel-air ratio for correct running of the engine.
Due to the pressure drop developed across the throttle plate a partial vacuum is formed around the throttle plate disc, thus causing the atomised fuel to turn to vapour, thereby cooling the fuel air mixture by the latent heat of vaporisation.
The cooling effect of the vaporisation of the fuel is instantaneous and significant. It sets up a temperature gradient of 20-30° C. around the throttle plate and under certain relative humidity conditions and intermediate throttle angles, and hence air flow rates, ice will rapidly form on the throttle plate and associated metal parts of the carburetor. This partial obstruction of the airflow, due to ice formation, can lead to the fuel/air ratio altering to such an extent that the mixture becomes too rich and the engine will stop in a matter of seconds. This is a highly undesirable situation, especially for single engine aircraft. Hitherto, this problem has been addressed by either heating the carburetor body or heating the air before it enters the carburetor. These methods are a compromise and will not prevent icing under all conditions. These methods furthermore require large amounts of heat to effectively prevent icing, demanding a compromise between engine power and effective ice prevention. For this reason most designs require the system to be manually engaged by the pilot, as stated in the aircraft flight manual, and are susceptible to pilot error. Both of these methods can reduce the engine power by as much as 15-20% and as such are only engaged when maximum power from the engine is not essential.